1. Field of the Invention
The present invention relates to a method of limiting and localizing the penetration of an oil-base drilling fluid into porous underground strata, crevices, and fissures during the drilling of a well in a permeable subterranean formation.
2. Description of the Prior Art
In the drilling of a well into the earth by rotary drilling techniques, a drill bit is attached to a drill string and the drill bit is rotated in contact with the earth to cut and break the earth and form a well therein. A drilling fluid is circulated between the surface of the earth and the bottom of the well to remove drill cuttings therefrom, lubricate the bit and drill string, and to apply hydrostatic pressure to the well to control the flow of fluids into the well from the earth formation penetrated by the well.
Drilling fluids commonly employed include gaseous and liquid drilling fluids. Liquid drilling fluids, often called "drilling muds" may have a liquid base which is water or an oil-base which is a hydrophobic substance such as oil. This invention relates to the latter and includes, as is common in the art, a minor portion of water present in the oil as an emulsion of the water-in-oil (invert) type.
In the drilling of a well into the earth, a problem sometimes occurs which involves the loss of unacceptably large amounts of drilling fluid into strata, crevices, and fissures of a formation penetrated by a well. This is sometimes referred to as "lost circulation". Such a formation or portion thereof that accepts the drilling fluid is commonly termed a "lost circulation zone". Lost circulation may occur when the well encounters a formation of an unusually high permeability or one which has naturally occurring fractures or fissures. Lost circulation may also occur by the inadvertent fracturing of a formation during drilling operations. Such fracturing sometimes occurs when the density of the mud is increased to provide a sufficiently high hydrostatic pressure to control high formation pressures. Other inadvertent fracturing of formations sometimes occurs because of fluctuations of the hydrostatic pressure imposed on the formations due to movements of the drill string and well casing in a well.
In addition to lost circulation, filtration and/or seepage losses are commonly encountered drilling problems. Seepage is a problem primarily incurred while spudding in though it may continue during the entire drilling phase as well.
Oil-base drilling fluids are generally more expensive than water-base drilling fluids. This added expense is justified for a number of reasons. One such reason is that oil-base fluids prevent formation damage to water sensitive reservoirs and prevent hole enlargement and permit coring in water-soluble rocks, such as salt. In addition, oil-base drilling fluids maintain stable mud properties in deep and high temperature holes which require heavy mud. Further, these fluids prevent corrosion of the drill pipe, casing, and tubing which reduces expensive workovers. They usually allow an increased drilling rate because of better lubrication and removal of the shell cuttings without disintegration of the cuttings. Another advantage with oil-base drilling fluids is that they generally can be reclaimed for a later use and this reduces the overall cost.
It is desirable for a number of reasons to reduce the loss of a drilling fluid during a drilling operation. For instance, oil-base drilling fluids are expensive and their loss can appreciably increase the cost of a drilling operation. In addition, if the oil-base fluids wet the formation by leakage during the drilling operation, the result can lead one to believe that the formation is a producing formation.
The reduction in seepage loss of a drilling fluid has been a long standing problem with drilling fluids and various methods have been used in the past to alleviate this problem. For example, U.S. Pat. No. 4,222,444 discloses the use of magnetic material to reduce the amount of drilling fluid lost to a formation. Also U.S. Pat. No. 3,509,951 discloses the use of a wax emulsion to control the loss of a drilling fluid while drilling a well. In addition, fibrous materials have been used in drilling fluids in order to form a sheath on the formation surface and prevent the loss of the drilling fluid into the formation.